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Force
Newton's Laws of Motion First Law of Motion An object continues to be in its state of rest or uniform motion , until and unless an external unbalanced force acts on it . Second Law of Motion Force acting on an object is directly proportional to the rate of change of Momentum with respect to time . Third Law of Motion For every action , there is an equal and opposite reaction . This reaction is limiting and acts in opposite direction as long as it is capable of withstanding the force . Force Force = Mass x Acceleration Unit : Newton ; 1 Newton is the force required to hold a body of mass 102 g near the earth's surface . Forces in Nature 1) Gravitational Force 2) Electromagnetic Force 3) Strong Nuclear Force : A nucleus contains protons and neutrons . Protons being positively charged exert repulsive force on each other . But , this force is not enough to break the nucleus . This is because there exists a strong interaction inside the nucleus . This is called as strong nuclear force . 4) Weak Nuclear Force : Forces when reactions involving changes in protons , neutrons and electrons takes place . Law of Conservation of Momentum The law of conservation of momentum states that the total momentum of a system of objects is always conserved . Frame of Reference A Frame of reference is a fixed co-ordinate system , with respect to which the position and motion of an object is determined . Inertial Inertial frame of reference is a frame of reference in which Newton's First Law of Motion holds good . i.e. it s an unaccelerated frame of reference . Non-Inertial (uniformly accelerated for JEE) Non-Inertial Frame of reference is a frame of reference in which Newton's First Law of Motion does not hold good . i.e. It is an accelerated frame of reference . Since it is an accelerated frame of reference , even if no force is acting on the object , the object will be set in motion . e.g. A person sitting in train . Conservative and Non Conservative Force The Forces that conserve Total Energy , Kinetic Energy and Momentum are called as Conservative Forces . The Forces that conserve Total Energy and Momentum but not Kinetic Energy are called Non Conservative Forces . Moment of Force Moment of Force = Force x Moment arm Moment Arm is the distance through which the Force is acting . Couple & Torque Torque = Force x Distance between the two forces . Centre of Mass The fixed point of an object at which the entire mass of the object is supposed to be concentrated is called Centre of Mass . CM = m1r1 + m2r2 + m3r3 + ...+ mnrn / M Centre of Gravity The fixed point through which the entire weight of the object is supposed to act in the downward direction is called as Centre of Gravity . Conditions of Equilibrium The resultant force should be 0 ; i.e. it should not produce motion or acceleration . If the Forces in Equilibrium prevent rectilinear motion , it is called as translational equilibrium . If the forces in equilibrium prevent rotational motion , it is called rotational equilibrium . Tips & Tricks 1) Working with Newton's Laws of Motion ''' i. Select the system ii. Identify the forces iii. Make a free body diagram iv. Choose and write Equations '''2) Working with Tension in String 3) Horse and Cart : 4) If in any question , based on any chapter , direction is being asked , then always work with vectors (xi + yj + zk) Category:Physics